1. Field of the Invention
The present invention relates to an impregnated soft, flat gasket, particularly for use as a cylinder head gasket in internal combustion engines, and a manufacturing method therefor.
2. Discussion of the Prior Art
Soft gaskets suitable for use as cylinder head gaskets for internal combustion engines are generally composed of nonwoven fiber mats comprised of organic fibers, inorganic fibers, synthetic fibers, natural fibers or mixtures thereof, and binders, preferably elastomeric binders, such as rubber. The nonwoven fiber mats may be reinforced with one or more metal inserts imbedded in the interior, and the nonwoven fiber mats may additionally include a filler having a fine grain size. In order to increase the cross-sectional density and strength of such gaskets, U.S. Pat. No. 4,201,804 to Stecher et al--the contents of which are hereby incorporated by reference--discloses an impregnation of the nonwoven fiber mat with impregnating agents, which are cross-linkable liquids. Subsequently, the impregnating agent is partially or fully cross-linked, preferably in a thermal process.
The deformation behavior of impregnated gaskets in response to applied surface pressures is influenced by the impregnating agent. Compared to a non-impregnated fiber mat gasket, the impregnated fiber mat gasket begins to flow under lower applied surface pressures and, thus, is usable only for applications in which it is subjected to such lower surface pressure stresses. This is because the yield points of impregnated fiber mat gaskets are lower than the yield points of non-impregnated fiber mat gaskets. This low yield point is a drawback, particularly in a gasket which has openings framed by casings, because in the zones of encased gasket openings particularly high sealing pressures should prevail in the installed state for obtaining the highest possible sealing quality. Therefore, according to U.S. Pat. No. 3,970,322 to Stecher et al, gasket zones underneath the encased edges of gasket openings are sought to be kept free of impregnating agent. For this purpose, the gaskets are first encased and subsequently impregnated, preferably by immersion in a bath of impregnating agent. However, the casings, which have an approximately U-shaped cross section and are bent around the edges of the soft material, may spring open after mounting on the gasket and create a gap between the soft material and the outer circumferential edges of the casings. Thus, the impregnating agent may flow into the soft material underneath the casing during the immersion process. The gaskets may then have a disadvantageously low yield point underneath the encased edges. Further, for some applications, it is necessary for the gasket to have a high yield point over the entire impregnated surface.
It is further known to add anti-aging materials to the fiber mat to protect impregnated gaskets against aging which results from the impegnating agent becoming hard and brittle, particularly after longer periods of storage. Thus, as disclosed in German Patent No. 2,637,249, it has been found particularly advantageous to add to the fiber mat small amounts of phenol resins, such as, from 0.2 to 2 weight %, to retard aging. However, no increase in the yield point for the impregnated gasket was obtained.